1. Field of the Invention
The present invention relates to a method for separating and purifying 2,6-dimethylnaphthalene, and in particular, to a method for separating and purifying 2,6-dimethylnaphthalene, in which 2,6-dimethylnaphthalene of high purity is obtained from a mixture of dimethylnaphthalene isomers with a high yield, by means of a combined process of column melt crystallization and sweating operation.
2. Description of the Related Art
2,6-Dimethylnaphthalene (hereinafter, may be abbreviated to 2,6-DMN) is a raw material used for the production of fibers, films and the like made of polyethylenenaphthalate (PEN). Therefore, in order to produce a highly functional polyethylenenaphthalate, the raw material thereof, 2,6-dimethylnaphthalene, is also required to be a product of high purity.
In general, 2,6-dimethylnaphtahlene is produced by a series of reactions, starting from a reaction between o-xylene and butadiene followed by alkenation, cyclization, dehydrogenation and isomerization, and is finally obtained as an isomeric mixture rich in 2,6-dimethylnaphthalene. Dimethylnaphthalene (DMN) may exist in 10 different isomeric forms such as 2,6-DMN, 2,7-DMN, 2,3-DMN, 1,2-DMN, 1,3-DMN, 1,4-DMN, 1,5-DMN, 1,6-DMN, 1,7-DMN, and 1,8-DMN, depending on the positions of two methyl groups in the structure. Accordingly, in order to obtain 2,6-dimethylnaphthalene of high purity, a process of separating and purifying 2,6-dimethylnaphthalene from a mixture of dimethylnaphthalene isomers is required.
For the method for separating and purifying 2,6-dimethylnaphthalene widely used at present, such methods are known: 1) a method for separation by crystallization; 2) a method for separation by adsorption; 3) a method for forming a complex of 2,6-dimethylnaphthalene with a certain type of organic compound, separating this complex, and then decomposing the complex; and so on.
The techniques known in the related art, which are pertinent to the method for separating 2,6-dimethylnaphthalene, include the following.
Korean Patent No. 10-0463076 describes a method for separating 2,6-dimethylnaphthalene of high purity by selectively separating a mixture of dimethylnaphthalene isomers containing 2,6-dimethylnaphthalene from a naphthalenic mixture containing dimethylnaphthalene isomers through recrystallization, fractionation or the like, and then crystallizing 2,6-dimethylnaphthalene under pressure in the presence of a solvent. However, the dimethylnaphthalene isomers have very close boiling points, as shown in the following Table 1, and thus, it is difficult to separate and purify 2,6-dimethylnaphthalene by distillation.
TABLE 1DMN isomerMelting point (° C.)Boiling point (° C.)1,6-DMN−16.02661,7-DMN−14.02631,3-DMN−4.22651,2-DMN−3.52711,4-DMN6.02651,8-DMN65.02701,5-DMN82.02692,7-DMN98.02622,3-DMN104.02692,6-DMN112.0262
Furthermore, as shown in Table 1 above, the melting point of 2,6-dimethylnaphthalene is the highest among the isomers of 2,6-dimethylnaphthalene. Thus, it is possible to separate and purify 2,6-dimethylnaphthalene by melt crystallization.
Korean Laid-Open Patent Application No. 10-2001-33746 describes a method for producing 2,6-DMN of high purity with a high yield from a mixture of DMN isomers through a series of processes including fractionation, crystallization and adsorption, without restricting 2,6-DMN, which is to be used in the production of polyethylenenaphthalate, to a specific isomer present in the raw material supplied. The above method is characterized in that 2,6-DMN is dissolved in p-xylene and o-xylene as final purifying step through crystallization, to adsorb and separate it.
Japanese Laid-Open Patent Application No. 1997-249586 and Japanese Laid-Open Patent Application No. 1997-301900 describe methods for producing 2,6-DMN from a mixture of DMN isomers through crystallization in the presence of a solvent. These methods are directed to industrially advantageous means of separation and recovery, because the methods allow maintaining of the product purity at at least a predetermined level with stability over a long period of time.
U.S. Pat. No. 5,675,022 describes a method for dynamic melt crystallization using a Sulzer Chemtech apparatus, which is a falling film crystallizer, comprising flowing a molten liquid on a cooled surface, in the form of a liquid film, by means of forced convection. However, this method involves dynamic layer crystallization which disadvantageously requires performing crystallization 5 times or more through multi-stage (5-stage) crystallization, and use of additional apparatuses.
Korean Patent No. 10-0463076 describes a method for separation and purification to obtain 2,6-diemthylnaphthalene of high purity with a high yield, by separating 2,6-dimethylnaphthalene of high purity from residue oil of naphtha cracking and a mixture of dimethylnaphthalene isomers, through a combined process of melt crystallization and extractive crystallization. However, this method is operated in a batch mode, and thus has limitations in the throughput, and difficulties in scaling up, and the method is not suitable for the separation in an industrial scale.
Korean Patent No. 10-0100533 describes an industrially advantageous method for separating 2,6-dimethylnaphthalene of high purity from a feed material containing a mixture of dimethylnaphthalene isomers, by conducting separation by adsorption using an adsorbent comprising zeolite Y containing an alkali metal or zinc, and a desorbent containing 60% by weight or more of p-xylene or o-xylene.
Among these methods, the methods employing crystallization are known to be simplest and most suitable for industrial application. However, the methods employing crystallization have problems of requiring relatively high sums of fixed investment and production costs because the process is relatively complicated, results in a low yield, and makes use of expensive solvents. Particularly, in the case of using a separation process through crystallization, the separation process involves simple cooling and crystallization in most cases, and is focused on the process of isomerization or adsorption using catalyst, rather than crystallization.